The carcinogenic effects of 26 substances, including antineoplastic and immunosuppressive agents, contaminants of human foodstuffs, rodent carcinogens, pesticides, and artificial sweeteners are being evaluated in four species of non-human primates. Seventeen of these substances have not as yet demonstrated carcinogenic activity, although some have been on test for less than three years. Nine of the test compounds are carcinogenic in non-human primates, producing tumors in 9.1-100% of the treated animals. The effects of six of the nine compounds (DENA, DPNA, 1-nitrosopiperidine, aflatoxin B1, MAM-acetate and urethane) were manifested primarily as hepatocarcinogenicity. 1-methyl-1-nitrosourea induced squamous cell carcinomas of the oropharynx and esophagus, with the esophageal tumors possessing clinical and morphologic similarities to human esophageal carcinoma. Long-term treatment with procarbazine resulted in an increased incidence of malignancies, approximately one-half of which were AML. One of 10 monkeys treated with adriamycin also developed AML. Alpha-fetoprotein has been found to be a useful marker for diagnosing and for following the treatment of hepatocellular carcinoma in non-human primates. Studies on chemo-prevention of cancer using various nucleophiles are in progress.